Seismic while drilling system and methods

ABSTRACT

A bottom hole assembly is configured with a drill bit section connected to a pulse generation section. The pulse generation section includes a relatively long external housing, the particular housing length being selected for the particular drilling location. The long external housing is positioned closely adjacent to the borehole sidewalls to thereby create a high speed flow course between the external walls of the housing and the borehole sidewalls. The long external housing includes a valve cartridge assembly and optionally a shock sub decoupler. While in operation, the valve cartridge assembly continuously cycles and uses downhole pressure to thereby generate seismic signal pulses that propagate to geophones or other similar sensors on the surface. The amount of bypass allowed through the valve assembly is selectable in combination with the long external housing length and width to achieve the desired pulse characteristics. The bottom hole assembly optionally includes an acoustic baffle to attenuate wave propagation going up the drill string.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Non-Provisional Patent Application, Ser. No. 13/870,866 filed on April 25, 2013, which claims priority to U.S. Provisional Patent Application, Ser. No. 61/638,456 filed on Apr. 25, 2012 and U.S. Non-Provisional Patent Application, Ser. No. 12/957,049 filed on Nov. 30, 2010, which issued as U.S. Pat. No. 8,328,649 on Sep. 10, 2013, each incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to seismic pulse generation and downhole drilling operations.

2. Description of the Related Art

Downhole drilling tool assemblies including mud pulsers that transmit data to surface sensors on the drillstring have seen use in drilling operations for some time. At least one benefit of including seismic sources in downhole drilling assemblies is the ability to drill while collecting seismic data which allows real-time seismic data collection and processing that in turn allows for real-time decision making based on current seismic information.

Seismic while drilling operations have been based on the noise and vibration generated by the drill bit during the drilling process, utilizing this noise and vibration as the seismic source. This method requires the use of a roller cone bit that imparts continuous small impacts to the rock surface allows reverse vertical seismic profiling (rVSP) while drilling, which provides a measure of the travel time of a pressure wave (P-wave) from the source to the surface. A number of sensors, such as geophones, are deployed at a number of locations on the surface and listen to or receive the noise and vibration generated by the downhole drill bit as the noise and vibration propagates to the surface. Roller cone bits do not generate a strong seismic signal in soft formations and PDC bits, which are more commonly used for horizontal drilling, do not generate a seismic signal at all.

U.S. Pat. No. 7,139,219 describes near-drill-bit source that can produce seismic signals regardless of bit selection or formation type. This source has a pulse duration of a few milliseconds and swept frequency cycle rate of 10 to 20 Hz. The swept cycle rate eliminates timing ambiguity in signal processing for reverse vertical seismic profiling and allows the seismic signal to be stacked over time to generate a stronger signal. The swept-frequency device incorporates mechanical complexity and generates a seismic signal with limited range.

The hydraulic pulse valve assembly described in U.S. Pat. No. 8,939,217, is capable of generating pressure pulses in the borehole annulus and drill pipe upstream of the BHA with a rise time on the order of one millisecond. This invention incorporates pulse control features that allow tailoring of the pulse width over the range of 5 to 100 milliseconds and adjustment of the pulse repetition cycle rate over the range of 1 to 10 Hz. Once configured, the cycle rate of the tool at a given flow rate is fixed.

In addition to P-waves, drilling operations may also generate shear waves (S-waves) that generally propagate in toroidal pattern away from the drill bit. Seismic while drilling methods have not used S-waves because those impulses are generally not strong enough to be reliably detected at the surface. Thus, it would be desirable to generate a repeated strong impulsive axial point load at the bit face to generate a shear wave (S-wave) for seismic interpretation while drilling. The signals should be detectable using a microseismic surface array while drilling a typical horizontal well with a depth on the order of 3000 meters.

Additionally, it would be desirable to simultaneously generate a strong pressure (P-wave) to enhance interpretation capabilities. The ratio of P- to S-wave velocity is sensitive to pore pressure in the formation while the S-wave amplitude and radiation pattern is sensitive to the presence and orientation of natural fracture networks. Natural fracture networks provide the primary reservoir storage and production pathways for oil and gas in shale formations. When these reservoirs are completed through hydraulic fracturing, only a few of the zones typically provide the bulk of production. The location of natural fracture networks while drilling the horizontal well would allow fracture stimulation to focus on only the potentially productive zones, providing significant economic and environmental benefits from reduced water and sand usage.

Operation of a tool at low frequency would generate seismic energy that propagates long distances. Ideally the S- and P-wave impulses would repeat at a rate in the range of 1 Hz to 10 Hz to maximize the range at which the signals will propagate without interference with mud pulse telemetry. The impulsive source pulse width would ideally be long enough to provide strong seismic coupling to the formation but short enough that discrete impulses are received at surface and do not interfere with each other.

SUMMARY OF THE INVENTION

In accordance with the invention, a strong shear (S) and pressure (P) wave source for seismic while drilling is deployed along with a surface seismic array. This innovation provides for improved seismic data and also can enhance an operator's ability to detect natural fracture networks. A downhole drilling tool assembly (bottom hole assembly or BHA) having a drill bit section, and a pulse generation section, in accordance with the invention, can be configured to achieve these characteristics. The seismic signal can be processed in various ways. The simplest technique is to monitor the amplitude of the S-wave signal received at surface using at least one but preferably an array of 3-axis (3D) seismometers. A more refined approach would compare the ratio of the S- and P-wave signals to account for other variables that could affect coupling of the seismic signal to the formation. An array of 3D sensors would allow a determination of the preferred orientation of the fracture networks. Finally the difference in phase between the S- and P-wave will allow evaluation of changes in pore pressure since the ratio of the velocity of these waves is a strong measure of pore pressure.

In one embodiment of the invention designed to maximize P-wave generation, a bottom hole assembly is configured with a drill bit section coupled to a water hammer pulse generator. The pulse generation section includes a relatively long external housing near the drill bit with enlarged diameter relative to the drillstring with each side of the housing closely adjacent to the borehole sidewalls, the particular housing diameter and length being selected for the particular drilling location. The typical length would be on the order of 10 meters. The water hammer valve may be located near the bit or some hundreds to a thousand meters upstream of the bit. A decoupling assembly may be located between the water hammer valve and the bit in order to improve mechanical coupling of the water hammer pulse inside the drillstring to the bit face and into the formation.

While in operation, the valve cartridge assembly continuously cycles and modulates flow though the tool to thereby generate seismic signal pulses that propagate to geophones or other similar sensors on the surface. The cycle rate characteristics are determined by sizing flow restrictions within the pulse valve disclosed in U.S. Pat. No. 8,939,217 and by varying the length of the pilot and piston components described in this patents In particular, the cycle rate can be selected for a particular application to be anywhere between 1 and 10 Hz.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 A is a side elevational view of a seismic while drilling system for detecting fracture networks showing ground motions while drilling though an unfractured formation.

FIG. 1B is a side elevational view of a seismic while drilling system for detecting fracture networks showing ground motions while drilling though a fractured formation

FIG. 2 is a side elevational view of an exemplary horizontal drilling system bottom hole assembly of FIG. 1.

DETAILED DESCRIPTION

Exemplary embodiments are illustrated in referenced Figures of the drawings. It is intended that the embodiments and Figures disclosed herein are to be considered illustrative rather than restrictive. No limitation on the scope of the technology that follows is to be imputed to the examples shown in the drawings and discussed herein.

An embodiment of the bottomhole assembly of a seismic-while-drilling system for detecting fracture networks system is shown in FIG. 1A. This embodiment incorporates a water hammer pulse generator as part of a drilling bottomhole assembly and a drill bit as shown in FIG. 2. The water hammer pulse generator may be located near the bit or located some distance upstream in order to minimize shock loading on the measurement-while-drilling system that is typically included in the BHA. Even if located some distance away from the drill bit, the pulse generator is still considered to be part of the BHA. FIG. 1 shows the system deployed in a horizontal well, however any well configuration may be used. FIG. 2 shows a shock subassembly above the water hammer valve and which acts to decouple the water-hammer valve from the drillstring to improve mechanical coupling to the formation.

The seismic source generates both a pressure impulse due to pressure changes in the annulus and a shear wave due to the impact of the bit on the hole bottom. The shear energy radiates in a toroidal pattern around the bit as shown in FIG. 2. The shear waves propagate radially from the bit as shear waves that are polarized in the plane that contains the borehole axis and the surface receiver. For receivers located directly over the source, the shear waves are vertically polarized and the ground motion is parallel to the borehole axis. In the simplest case with receivers directly above the source the motion due to the pressure wave is vertical and the motion due to a shear wave is horizontal. The water hammer pulse generator is capable of producing S-waves that are strong enough and consistent enough to be reliably detected at the surface. This provides several advantages and benefits, as discussed in more detail below.

The BHA may incorporate a directional-drilling MWD system that continuously transmits the depth, inclination and azimuth of the borehole. Because this information is known, the position of each seismic receiver in relation to the source is also known, as is the direction of the ground motion due to the shear wave and pressure waves. In a uniform medium, the pressure wave motion is the component of motion along a radial vector from the source. Most sedimentary formations exhibit layering and refraction, which will cause the ray path to deviate from a straight line but the P-wave motion will still be dominantly radial from the source. The first arrival of the P-wave can be used to determine the actual direction of motion instead of relying on the bit location derived from the MWD system. The shear wave motion is normal to the pressure wave motion and lies in the plane that contains the borehole axis. The water hammer pressure pulse in the annulus is pressure reduction so the initial motion of the pressure wave is towards the source. Those skilled in the art will recognize that the shear waves will be highly attenuated by the presence of a natural fracture network, while the pressure waves will be attenuated to a lesser extent. The relative amplitude of the shear wave to the P wave is thus a clear indication of the presence of fracture networks in the formation surrounding the source. Thus, if the ratio of S-wave to P-wave is lower in one portion of the wellbore than in another portion, that would be indicative of a fracture network. The ability to reliably detect S-waves at the surface, which is enabled by the water hammer pulse generator, is what allows this data to be used to detect the location of fracture networks.

The pulse generator signal is repeated continuously and may be autocorrelated to accurately determine the cycle rate. The signal from repeated pulses may then be stacked to improve the signal to noise ratio.

The shear wave amplitude will tend to be smaller than the pressure wave amplitude and the arrival time of the shear will be delayed. The pressure wave signal may be used as a pilot signal that is cross-correlated with the shear wave signal to improve the signal to noise ratio and amplitude measurement accuracy. This process also allows monitoring of the phase lag between the two signals.

FIG. 1 shows only a single 3-axis receiver on surface however an array of surface receivers or receivers in boreholes may also be employed. The signal received from multiple receivers may be cross-correlated to determine the travel time difference of the signals and then time shifted and stacked to improve the signal to noise ratio and accuracy of the amplitude measurement.

In an embodiment, and as shown in FIG. 2 the external housing 50 contains a shock sub decoupler 60 and a valve cartridge assembly 70. This type of shock sub is known to those skilled in the art and is shown schematically. The shock sub 60 allows for a small amount of axial movement of the bottom hole assembly below the sub relative to the drill collars 72. Shock subs can be used to dampen drilling vibrations. The torsional load of drilling is transmitted though the shock sub 60 with a spline 74 coupling that is free to slide up and down within limits. A sliding seal 76 maintains the pressure of drilling mud in the tool. Drilling weight acts against the springs 78 causing them to compress while internal pressure acts to extend the sub 60. Pressure pulses produced by the valve cartridge assembly 70 will act to extend the sub 60, thereby absorbing upstream pressure pulses and converting the pulse energy to a downward force on the drill bit 20. This downward force acts as a monopole seismic source that primarily generates shear wave energy.

Although the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description. 

1. A seismic-while-drilling system comprising: a bottom hole assembly located at the end of a length of tubing inserted into a borehole for drilling, the bottom hole assembly comprising: a drill bit section, a motor section, and a pulse generation section comprising an external housing having a valve cartridge assembly configured to create repeated axial impulsive loads to impart force to the drill bit section; and at least one surface receiver configured to record ground motion due to shear waves radiated from the drill bit section.
 2. The seismic while drilling system of claim 1, further comprising a surface receiver configured to record ground motions due to pressure waves radiated from the drill bit section.
 3. The seismic while drilling system of claim 1 further comprising a shock sub decoupler configured between said valve cartridge assembly and said drill bit.
 4. The seismic-while-drilling system of claim 1 in which the pulse generation section is configured to cycle at a rate of 10 Hz or less.
 5. A method of detecting natural fracture networks in subsurface formations while drilling, comprising the steps of: running into a borehole a drillstring comprising: a bottom hole assembly comprising a drill bit section and a pulse generation section; drilling with the drill bit section of the bottom hole assembly; generating shear wave impulses at the drill bit section by generating pulses with the pulse generation section; recording surface motion due to shear waves radiated from the drill bit section; and monitoring changes in shear wave amplitude as the borehole penetrates the formation.
 6. The method of claim 5, further comprising the steps of: generating pressure wave pulses with the pulse generator section recording surface motion due to pressure waves radiated from the drill bit section.
 7. The method of claim 6 further comprising the step of comparing the amplitude of said shear waves with said pressure waves.
 8. The method of claim 6 further comprising the step of comparing the arrival time of shear waves and pressure waves.
 9. The method of claim 5 further comprising the step of operating the pulse generation section at a frequency of 10 Hz or less. 